Method for forming an aluminum contact

ABSTRACT

A method for forming an aluminum contact through an insulating layer includes the formation of an opening. A barrier layer is formed, if necessary, over the insulating layer and in the opening. A thin refractory metal layer is then formed over the barrier layer, and aluminum deposited over the refractory metal layer. Proper selection of the refractory metal layer and aluminum deposition conditions allows the aluminum to flow into the contact and completely fill it. Preferably, the aluminum is deposited over the refractory metal layer without breaking vacuum.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to the formation ofcontacts in integrated circuits, and more specifically to a method forforming interlevel aluminum contacts.

[0003] 2. Description of the Prior Art

[0004] In semiconductor integrated circuits, formation of metalinterconnect layers is important to the proper operation of thesedevices. Metal interconnect signal lines make contact to lowerconductive layers of the integrated circuit, including the surface ofthe silicon substrate, through vias in an insulating layer. For bestoperation of the device, the metal used to form the interconnect layershould completely fill the via.

[0005] Because of its physical and electrical properties, aluminum isespecially suited for fabrication of metal interconnect lines inintegrated circuits. However, the sputtering process used to applyaluminum thin film layers to an integrated circuit generally results inless than ideal filling of contact vias. Large aluminum grains tend toform on the upper surface of the insulating layer. Those grains whichform at the edges of the contact via tend to block it before aluminumhas a chance to completely fill the via. This results in voids anduneven structures within the via.

[0006] This problem is especially acute as integrated circuit devicesare fabricated using smaller geometries. The smaller contacts used inthese devices tend to have a larger aspect ratio (height to width ratio)than larger geometry devices, which exacerbates the aluminum fillingproblem.

[0007] The uneven thickness of the aluminum layer going into the via,caused by the step coverage problem just described, has an adverseimpact on device functionality. If the voids in the via are largeenough, contact resistance can be significantly higher than desired. Inaddition, the thinner regions of the aluminum layer will be subject tothe well known electromigration problem. This can cause eventual opencircuits at the contacts and failure of the device.

[0008] Many approaches have been used to try to ensure good metalcontact to lower interconnect levels. For example, refractory metallayers have been used in conjunction with the aluminum interconnectlayer to improve conduction through a via. Sloped via sidewalls havebeen used to improve metal filling in the via. The use of slopedsidewalls is becoming less common as device sizes shrink because thesloped sidewalls consume too much area on a chip.

[0009] Even with these techniques, the problems of completely filling avia with aluminum are not solved. In part this is because aluminum isdeposited at temperatures which tend to encourage fairly large grainsizes. Voids and other irregularities within the contact continue to beproblems with current technologies.

[0010] One technique which has been proposed to overcome the via fillingproblem is to deposit the aluminum interconnect layers at a temperaturebetween 500° C. and 550° C. At these temperatures, the liquidity of thealuminum is increased, allowing it to flow down into the vias and fillthem. This technique is described, for example, in DEVELOPMENT OF APLANARIZED Al—Si CONTACT FILLING TECHNOLOGY, H. Ono et al, June 1990VMIC Conference proceedings, pages 76-82. This references teaches thattemperatures below 500° C. and above 550° C. result in degraded metalfilling of contact vias. It is believed that use of such a techniquestill suffers from problems caused by large grain sizes.

[0011] Another technique for improving metal contact step coverage isdescribed in U.S. Pat. No. 5,108,951 issued to Chen et al, entitledMETHOD FOR FORMING A METAL CONTACT. This patent describes a techniquefor depositing aluminum at low deposition rates within a specifiedtemperature range. The temperature is ramped up from a temperature belowapproximately 350° C. while aluminum is being deposited. The teachingsof this patent provide for deposition of the majority of the depth ofthe aluminum layer at a temperature between approximately 400°-500° C.at relatively low deposition rates.

[0012] The teachings of the Chen patent provide improved step coveragedeposition for aluminum contacts. However, the described technique stillsuffers from random voiding, which is believed to be caused byrelatively large grain sizes, or initial film nucleation which aredeposited at the temperatures described.

[0013] Many other variations to the deposition of aluminum have beenproposed and used in integrated circuit devices. Until now, all havesuffered to some degree from less than ideal via filling. Because of thenature of the deposition process, it appears that relatively minormodifications in the technology used to form the aluminum interconnectcan have important effects on the end result. What is heretofore lackingis a complete process which adequately provides for complete aluminumfill of the contact via.

[0014] It would be desirable to provide a technique for depositingaluminum thin film layers on an integrated circuit so as to improvecoverage in contact vias. It is further desirable that such a techniquebe compatible with current standard process flows.

SUMMARY OF THE INVENTION

[0015] Therefore, according to the present invention, a method forforming an aluminum contact through an insulating layer includes theformation of an opening. A barrier layer is formed, if necessary, overthe insulating layer and in the opening. A thin refractory metal layeris then formed over the barrier layer, and aluminum deposited over therefractory metal layer. Proper selection of the refractory metal layerand aluminum deposition conditions allows the aluminum to flow into thecontact and completely fill it. Preferably, the aluminum is depositedover the refractory metal layer without breaking vacuum.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The novel features believed characteristic of the invention areset forth in the appended claims. The invention itself however, as wellas a preferred mode of use, and further objects and advantages thereof,will best be understood by reference to the following detaileddescription of an illustrative embodiment when read in conjunction withthe accompanying drawings, wherein:

[0017] FIGS. 1-5 illustrate a preferred method for forming interlevelaluminum contacts according to the present invention;

[0018]FIG. 6 illustrates an interlevel contact formed according to apreferred alternative method of the present invention; and

[0019]FIG. 7 is a diagram illustrating preferred process conditions forformation of an interlevel aluminum contact according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] The process steps and structures described below do not form acomplete process flow for manufacturing integrated circuits. The presentinvention can be practiced in conjunction with integrated circuitfabrication techniques currently used in the art, and only so much ofthe commonly practiced process steps are included as are necessary foran understanding of the present invention. The figures representingcross-sections of portions of an integrated circuit during fabricationare not drawn to scale, but instead are drawn so as to illustrate theimportant features of the invention.

[0021] As is well known in the art, the term “aluminum”, when referringto metal deposited as conductive interconnect for integrated circuits,typically means aluminum alloyed with small amounts of other materialsrather than pure aluminum. For example, up to a few percent of siliconand/or copper are typically alloyed with the deposited aluminum in orderto improve the physical characteristics of the interconnect layer. Otheralloys, using a small percentage of other materials, are well known inthe art. Consistent with this usage of the term, “aluminum” as usedherein is intended to apply to such typical alloys as well as purealuminum.

[0022] Referring to FIG. 1, a contact is to be formed to a conductivestructure in a substrate 10. As used in this description, the substrate10 may be an actual silicon monocrystalline substrate, or may refer toany number of conductive and insulating layers overlying such amonocrystalline substrate. Insofar as the present invention isconcerned, the same techniques can be applied to contacts to the siliconsubstrate, or to any other underlying conducting layers.

[0023] An insulating layer 12 is formed over the substrate 10 using wellknown techniques, and an opening 14 is formed in the insulating layer12. Device fabrication to this point is wholly conventional, and wellknown to those skilled in the art.

[0024] Referring to FIG. 2, a layer 16 of refractory metal, such astitanium, is formed over the device and in the opening. Physical vapordeposition (PVD) is preferably used. A layer of titanium nitride 18 isformed over the titanium layer, followed by a second titanium layer 20.The three layers 16, 18, and 20 are preferably deposited without avacuum break between layers, in a single or multiple chamber sputteringmachine. These layers are preferably deposited at a temperature betweenapproximately 50° and 500° C.

[0025] Referring to FIG. 3, the device is then subjected to a well knownrapid thermal processing (RTP) step in a nitridation atmosphere, whichconverts the upper titanium layer 20 to titanium nitride. This step ispreferably performed at a temperature between approximately 550° and850° C. This results in the thickened nitride layer 18 shown in FIG. 3.As is known in the art, a good barrier layer greatly enhances thequality and reliability of the contact, and this sequence of stepsprovides a superior barrier layer. Other techniques for forming a goodbarrier layer can be substituted into the process if desired.

[0026] The RTP step also causes the lower titanium layer 16 to form asilicide region 22 with silicon exposed in the bottom of the contactopening 14. This will be the case with a contact made to amonocrystalline silicon substrate, or other silicon layer having siliconatoms free to alloy with the titanium. If the underlying layer containsno silicon, such a silicide layer 22 will not, of course, be formed.

[0027] The wafer containing the device is then loaded into amulti-chamber sputtering machine. As shown in FIG. 4, a thin layer oftitanium 24 is deposited over the device, preferably at a relatively lowtemperature of about 0° to 375° C. The titanium layer 22 preferably hasa thickness of between approximately 50 Å and 600 Å. The thickness ofthis layer will depend primarily on the size and aspect ratio of theopening 14.

[0028] Referring to FIG. 5, the wafer is then moved into an aluminumdeposition chamber without breaking vacuum. A thin layer of aluminum ispreferably applied at a low temperature of about 0° to 300° C. using aconventional sputtering method. This layer preferably has a thickness ofbetween about 500 Å and 2500 Å. The wafer is then moved into anotherchamber, having an elevated temperature of approximately 400° to 550°C., without breaking vacuum. Aluminum is then deposited immediately witha relatively slow deposition rate, preferably between approximately 20and 50 Å/sec. This results in a thicker aluminum layer 26, which fillsthe opening 14 and forms a planar layer over the entire chip. Thethickness of this layer 26 preferably ranges from about 2500 Å to thefull thickness of the layer to be deposited. As is known in the art,this thickness can have a wide range depending on device designconsiderations, and typically is about 5000 Å to 10,000 Å. After about2000 Å to 7000 Å have been deposited, the deposition rate can beincreased if desired. By this point, the opening 14 has beensubstantially filled, and a faster deposition rate will have little orno effect on the ultimate planarity of the aluminum layer above thecontact.

[0029] Alternatively, the wafer can remain in the same chamber, and thetemperature can be ramped up from the initial deposition temperature tothe final deposition temperature while aluminum is being deposited. Itwill be apparent to those skilled in the art that the use of a separatechamber has advantages in that no single chamber has to have itstemperature ramped up and down, which increases the overall throughputof the machine.

[0030] The aluminum layer 26 forms a very planar layer in large partbecause of the thin titanium layer which was formed immediately prior tothe aluminum deposition. This titanium layer acts to wet the surface ofthe wafer, increasing the surface mobility of the aluminum as it isdeposited. The thin titanium layer alloys with the aluminum layer toform an aluminum/titanium alloy layer 28, with the original titaniumlayer 24 being substantially completely consumed. If the titanium layer24 is relatively thick, only the upper portions will be converted toaluminum/titanium alloy.

[0031] Other refractory metals can be used in place of titanium, buttitanium appears to provide superior results in terms of planarizationand barrier formation. The formation of the thin barrier is preferablyin situ, with no exposure to air between deposition of the thin metallayer 24 and the overlying aluminum layer 26. This appears to enhancethe wetability of the aluminum as it is deposited over the underlyinglayers, improving the surface mobility of the aluminum and causing it topreferentially migrate into the opening and form a planar surface evenwhile filling the opening. It is believed that even a small amount ofoxide forming on the thin titanium layer interferes with this process,so the in situ deposition is strongly preferred.

[0032] Referring to FIG. 6, an alternative deposition technique isillustrated. This approach is suitable for use with second level metaldeposition, and in other instances where the contact is not made to anunderlying silicon layer. In these cases, it may not be necessary toform the barrier layer described above, formed from layers 16, 18, and20. Thus, the structure shown in FIG. 6 illustrates the formation of thethin titanium layer 24 directly on the insulating layer 12 and in theopening. This layer is consumed as before to form a thinaluminum/titanium alloy layer 28 underneath the aluminum. As before, forthicker layers of titanium, only the upper portions of the layer will beconverted to alloy, leaving a thin layer of relatively pure titaniumbeneath. In the alternative, a barrier of refractory metal (not shown inFIG. 6) can be deposited before the thin titanium layer 24, and it isnot necessary that this layer be a superior barrier layer as was thelayer described above in connection with FIG. 3.

[0033] The diagram of FIG. 7 illustrates the preferred processconditions obtained during deposition of the aluminum layer. After thewafer is moved into the second (heated) chamber for aluminum deposition,the temperature and deposition rate preferably fall within the outlineof the diagram. In the alternative embodiment, in which the wafer isheated while still in the first aluminum deposition chamber, the diagramillustrates the ranges of temperatures and depositions rates which isreached after the heating of the wafer is completed. Other depositionrates and temperatures may be used, especially after several thousandangstroms of aluminum have been deposited, at which point the openingshould be substantially filled with aluminum.

[0034] After deposition of the aluminum layer, processing of the deviceproceeds in accordance with known prior art principles. The aluminumlayer is patterned and etched to define interconnect. Furtherinterconnect layers can be formed at higher levels, using the describedtechniques or prior art approaches. Because the aluminum layer isextremely planar, even over the contact, it is possible to stackcontacts directly on top of each other without difficulty.

[0035] The described method, and resulting structure, results in asuperior aluminum contact which completely fills the opening and isplanar above it. Although many parts of the method are similar toprevious techniques, the unique combination of steps and conditionsdescribed above results in a reproducible, manufacturable contact whichis notably superior to those previously obtainable in the prior art.This is especially true for the manufacture of increasingly smallcontact openings.

[0036] While the invention has been particularly shown and describedwith reference to a preferred embodiment, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method for forming an interlevel aluminumcontact for an integrated circuit device, comprising the steps of:forming an opening through an insulating layer to expose a conductingstructure; forming a barrier layer over the insulating layer andextending into the opening to cover the conducting structure; forming athin layer of refractory metal over the barrier layer; and depositingaluminum over the thin refractory metal layer at a temperature and ratewhich maximizes surface mobility of the aluminum, allowing it tocompletely fill the opening.
 2. The method of claim 1, wherein the stepof depositing aluminum comprises the steps of: forming a first aluminumlayer at a temperature of less than approximately 375° C.; increasingthe temperature of the device to a temperature between approximately400° C. and 550° C.; and during the temperature increasing step,continuing to deposit aluminum over the first aluminum layer at a ratebetween approximately 30 and 50 Å/sec.
 3. The method of claim 2, furthercomprising; after the continuing to deposit step has depositedapproximately 2000 Å of aluminum, increasing the deposition rate.
 4. Themethod of claim 2, wherein the forming a first layer step is performedin a first chamber of a multi-chamber sputtering machine, and furthercomprising the step of: moving the device from the first chamber to asecond chamber of the multi-chamber sputtering machine without exposingthe device to air, wherein the second chamber is maintained at atemperature between approximately 400° C. and 550° C., wherein the stepof increasing the temperature is performed by the step of moving thedevice into the second chamber.
 5. The method of claim 1, wherein thestep of forming the barrier layer comprises the steps of: forming afirst refractory metal layer over the insulating layer and oversidewalls and a bottom of the opening; forming a refractory metalnitride layer over the first refractory metal layer; forming a secondrefractory metal layer over the refractory metal nitride layer; andheating the device in a nitridation atmosphere to convert the secondrefractory metal layer to a refractory metal nitride.
 6. The method ofclaim 5, wherein the first and second refractory metal layers comprisetitanium.
 7. The method of claim 6, wherein the thin layer of refractorymetal formed over the barrier layer comprises titanium.
 8. The method ofclaim 7, wherein the step of depositing aluminum causes the thin layerof titanium to alloy with the aluminum, forming a layer ofaluminum/titanium alloy between the aluminum layer and the barrierlayer.
 9. The method of claim 1, wherein the thin refractory metal layercomprises titanium.
 10. A method for forming an interlevel aluminumcontact for an integrated circuit device, comprising the steps of:forming an opening through an insulating layer to expose a conductingstructure; forming a thin layer of refractory metal over the barrierlayer; and depositing aluminum over the thin refractory metal layer at atemperature and- rate which maximizes surface mobility of the aluminum,allowing it to completely fill the opening.
 11. The method of claim 10,wherein the step of depositing aluminum comprises the steps of: forminga first aluminum layer at a temperature of less than approximately 375°C.; increasing the temperature of the device to a temperature betweenapproximately 400° C. and 550° C.; and during the temperature increasingstep, continuing to deposit aluminum over the first aluminum layer at arate between approximately 30 and 50 Å/sec.
 12. The method of claim 11,further comprising; after the continuing to deposit step has depositedapproximately 2000 Å of aluminum, increasing the deposition rate. 13.The method of claim 11, wherein the forming a first layer step isperformed in a first chamber of a multi-chamber sputtering machine, andfurther comprising the step of: moving the device from the first chamberto a second chamber of the multi-chamber sputtering machine withoutexposing the device to air, wherein the second chamber is maintained ata temperature between approximately 400° C. and 550° C., wherein thestep of increasing the temperature is performed by the step of movingthe device into the second chamber.
 14. A method for forming aninterlevel aluminum contact for an integrated circuit device, comprisingthe steps of: forming an opening through an insulating layer; depositinga first titanium layer over the insulating layer and in the opening;depositing a titanium nitride layer over the first titanium layer;depositing a second titanium layer over the titanium nitride layer;heating the device in a nitridation atmosphere, wherein the secondtitanium layer is converted to titanium nitride; depositing a thintitanium layer over the converted titanium nitride; depositing aluminumover the thin titanium layer; during the aluminum depositing step,heating the device to a temperature in the range of approximately 400°C. and 550° C., wherein the aluminum fills the opening and forms aplanar upper surface over the opening, and wherein the thin titaniumlayer alloys with the aluminum during deposition to form a layer ofaluminum/titanium alloy.
 15. The method of claim 14, wherein the thintitanium layer has a thickness of between approximately 50 Å and 600 Å.16. The method of claim 14, wherein the steps of depositing aluminum andheating the device comprise the steps of: depositing a first portion ofthe aluminum over the thin titanium layer at a temperature belowapproximately 375° C.; increasing the temperature of the device towithin the range of approximately 400° C. to 550° C.; and continuing todeposit aluminum during the step of increasing the temperature.
 17. Themethod of claim 16, wherein the device remains in a vacuum between thestep of depositing the thin titanium layer and the step of depositing afirst portion of aluminum over the thin titanium layer.
 18. The methodof claim 16, wherein the step of depositing a first portion of thealuminum is performed in a first chamber of a multi-chamber sputteringmachine, wherein the steps of increasing the temperature and continuingto deposit aluminum are performed in a second chamber of themulti-chamber sputtering machine, and wherein the first chamber ismaintained at a temperature below approximately 375° C. and the secondchamber is maintained at a temperature between approximately 4000° C.and 550° C., and further comprising the step of: after the step ofdepositing a first portion of aluminum, moving the device from the firstchamber to the second chamber without exposing the device to air. 19.The method of claim 18, wherein the step of depositing the thin titaniumlayer is performed in a third chamber of the multi-chamber sputteringmachine, and further comprising the step of: after the thin titaniumlayer is deposited, moving the device from the first chamber to thesecond chamber without exposing it to air.
 20. The method of claim 18,further comprising the step of: after the continuing to deposit aluminumstep has deposited aluminum to a thickness greater than approximately2500 Å, increasing the rate of deposition to a rate greater than 50Å/sec.
 21. A contact structure for a semiconductor integrated circuit,comprising; a conductive structure; an insulating layer overlying theconductive structure, the insulating layer having an opening therein toexpose a portion of the conductive structure; a layer oftitanium/aluminum alloy overlying a portion of the insulating layer andextending into the opening; and an aluminum layer overlying the layer oftitanium/aluminum alloy and extending into the opening, wherein thealuminum layer completely fills the opening and has a planar uppersurface over the opening.
 22. The contact structure of claim 21, furthercomprising: a barrier layer between the layer of titanium/aluminum alloyand the insulating layer, the barrier layer having a lower layer oftitanium, and an upper layer of titanium nitride.